Chapter 3 – Review KEY

1.  Explain what the formula of water, H2O tells us in your own words.

For every 1 oxygen atom, 2 hydrogen atoms the molecule, (ratio 2 to 1)

2.  What is the difference between 2N and N2?

A big 2 stands for “two” molecules of whatever is after it. A little 2 stands for both stuck together.

3.  Explain Dalton’s “Law of Constant Composition.”

Constant ratio throughout substance

4.  What did Dalton’s atom look like?

A round ball (very small)

5.  List the 5 main ideas of Dalton’s atomic theory. Which one of these is no longer accepted?

See book for all five, but not all atoms of an element are identical (Isotopes).

6. Pick the true statement, and explain why it is true.

An ion is formed

a.  by either adding or subtracting protons from the atom

false, changes element

b.  by either adding or subtracting neutrons from the atom

false, changes isotopes

c.  by either adding or subtracting electrons from the atom

true, it’s the only item that can be added or subtracted

d.  all of the above

e.  two of the above

7. A certain ion with 27 electrons has a charge of 2+. What ion is it?

a. Mn2+ b. Co2+ c. Cu2+

Explain your choice.

Cu has 29 e-, Cu +2 (remove 2 e-) has 27e-

8. An element forms an ionic compound with chlorine having the formula XCl2 (the charge of X in the compound is a +2). The ion of element X has a mass number of 89 and 36 electrons. Identify the element X and tell how many neutrons it has.

36e- ion à 38e- atom must be Sr à p+n = 89 with 38 p, Sr with 51 neutrons

9. Which of the following did Marie Curie study? Why was her work so important in determining the structure of the atom?

a. isotopes b. radioactivity c. Brownian motion d. electrons e. electricity

Radeoactivity. Since radioactivity is an atom breaking down, the process yields a lot of information about what is in the atom.

10. How did JJ Thomson discover the electron?

With the cathode ray tube. Atoms emit tiny particles, and when put through an electric field, it is attracted to the positive side so they must be (-) charge.

11. What part of the atom did Rutherford’s Gold Foil experiment discover? How?

Nucleusàpositive alpha particles thrown at gold foil ricocheted and bounced back.

12. Knowing the number of protons of a neutral atom enables you to determine which of the following (there may be more that one answer)

a. the number of neutrons b. the number of electrons

c. the name of the atom

What other information would you need to find the remaining items?

·  Mass number

1.)  Explain how the atoms of one element differ from those of another element.

For sure: different number of protons

In neutral atom: different number of electrons

Could have different number of neutrons

2.)  In what way are two isotopes of the same element? Explain why isotopes of the same element have the same chemical behavior.

Same: Exact same number of protons and electrons. Different: different number of neutrons

Chemical behavior: chemical behavior depends on electrons

3.)  Complete the Table:

Atomic Number / Mass Number / Number of Protons / Number of Neutrons / Number of Electrons
8 / 16 / 8 / 8 / 8
7 / 14 / 7 / 7 / 7
20 / 41 / 20 / 21 / 20
11 / 23 / 11 / 12 / 11
26 / 56 / 26 / 30 / 26

4.)

Silicon
mass number / exact mass of isotope (amu) / percent abundance
28 / 27.976927 / 92.230
29 / 28.976495 / 4.6700
30 / 29.973770 / 3.1000
The answer for silicon: 28.086

Using the atomic weight and percent abundance data for the isotopes of Silicon in the table above, calculate the average atomic mass of Silicon.

4.)  There are four naturally occurring isotopes of the element chromium. The relative abundance of each is: 50Cr = 4.31%, 52 Cr = 83.76%, 53Cr = 9.55%, 54Cr = 2.38%.

Calculate the average atomic mass (atomic weight) of chromium.

Mp = / 1.00728 amu
Mn = / 1.00866 amu
Me = / 5.48 x 10-4 amu

.0431[24(1.00728) + 26(1.00866) + 24(5.48x10^-4)] + 0.8376[24(1.00728) + 24(5.48x10-4) + 28(1.00866)] + .0955[24(1.00728) + 24 (5.48x10-4) + 29(1.00866)] + 0.0238[24(1.00728) + 24(5.48x10-4) + 30(1.00866)]

= 52.49 amu